"Castro has a little problem but he can live another 100 years with this little problem," Chavez was quoted as saying by Venezuela's Bolivarian News Agency.
Castro, 81, has not appeared in public since announcing more than a year ago that he had undergone intestinal surgery and was provisionally ceding power to his younger brother Raul.
Chavez, a close Castro ally, said Castro was "close to death" but underwent several operations and has regained weight.
Castro had "three operations, and he's 81, imagine that. They changed almost all the blood with transfusions. Fidel is alive because he is Fidel," Chavez said.
Chavez made the comments early Friday after meeting with Brazilian President Luiz Inacio Lula da Silva and Ecuadorean President Rafael Correa in the jungle city of Manaus.